1. Field of the Invention
The present invention relates to a method for fabricating a silica glass by a sol-gel process.
2. Description of the Related Art
In general, silica glass is transparent and chemically inert, and has high a level of thermal stability and strength, and a low thermal expansion coefficient. Because of such characteristics, silica glass has been useful for an optical device such as an optical fiber or optical lens.
Basically, an optical fiber is comprised of a core at the center thereof, and a cladding having a refractive index different from the core such that light is totally reflected from the core. In order to fabricate optical fibers, an optical fiber preform including a core rod and an overcladding tube enclosing the core rod is prepared. Then, the optical fiber preform is thermally-treated and then extended to form the optical fibers.
The optical fiber preform is prepared by a modified chemical vapor deposition (MCVD), vapor phase axial deposition (VAD) or outside vapor deposition (OVD) method.
In MCVD, the optical fiber preform is prepared from an overcladding tube made of high purity silica glass. Here, the silica glass overcladding tube is formed by a sol-gel process.
The productivity can be increased and the composition of the product can be controlled when using a sol-gel process as a liquid phase process. Also, because the sol-gel process is performed at a low temperature, the sol-gel process is very economical. Also, since a high purity material is used as the starting material, this method is very useful for fabricating a photomask for semiconductor and a high purity silica glass.
Hereinafter, a method of manufacturing an overcladding tube formed of silica glass by the sol-gel process will be briefly described.
First, silica particles are dispersed in water to form a sol. The formed sol is subjected to an aging treatment for a predetermined time. Then, the resultant sol is poured into a mold for gelation. After gelation is completed, the gel is separated from the mold and then dried.
Then, the dried gel is thermally-treated to remove organic substances from the gel. Then, a reaction for eliminating hydroxy groups from the resulting gel and sintering are performed, thereby producing an overcladding tube made of silica glass.
The reactivity of the above-described sol-gel process depends on the gelation reaction temperature, composition of the sol, pH and solvent, and it is very difficult to maintain the reactivity within a favorable range by controlling such factors. Also, when drying the molded gel, many cracks can form, and shrinking and cracking occur during sintering. In order to solve these problems, a method using a drying control chemical additive (DCCA) or a polymer binder, or a redispersion method and supercritical drying method have been developed.
The method using DCCA minimizes local differences in the solvent's evaporation rate from the gel, thereby minimizing the differences in the local stress of the samples during drying. As a result, the gel becomes hard, thereby decreasing the generation of cracks.
According to the redispersion method, dried fine silica powder, i.e., fumed silica, is dispersed in water to form a sol, and then the sol is gelated. During gelation, the silica particles form an agglomerate due to hydrogen bonds. After drying the agglomerate, the dried agglomerate is subjected to a thermal process and a grinding process and then is redispersed in water. The redispersed product is gelated, molded, and then sintered.
However, the above methods are not effective in preventing cracking during the drying of the gel, and the preparing process is complicated.